Converting Hydraulic Resistance Energy of the System into Electricity

Introduction. Traditional non-renewable primary energy sources, which are very limited in resources, continue to play a dominant role in the Armenian economy. The active use of fossil energy sources has caused environmental problems such as air and soil pollution, water scarcity, and ecosystem degradation. Population growth, continuous household and industrial energy consumption should be taken into account. All this suggests that further quantitative (extensive) development of the energy industry is inexpedient. The use of renewable energy sources to replenish the overall energy balance of the country is an imperative of the time. This approach makes it possible to provide sustainable development while preserving the environment. Water and energy resources are essential for human life and are subject to regular economic, technological, demographic, and social impacts [1]. According to some estimates, 2–3 % of the world's electricity consumption comes from water systems (WS) [2], and pumping units are used in 80–90 % of cases. The costs of their maintenance are among the main operating costs of WS [2]. The concept of rational and efficient use of water resources and electricity is of strategic importance for sustainable development and climate change mitigation. Rational use of water and electricity is hampered by weak infrastructure and outdated operating procedures. This is especially true for developing countries. Moreover, in line with the Millennium Development Goals¹, there is a need for alternative solutions: the goal is to halve the number of people without sustainable access to clean water and basic sanitation. As renewable energy sources, it is proposed to put into operation small hydroelectric power plants (SHPP) in sections of the water supply network with local hydraulic resistance, subject to semi-open valves or pressure regulators. This is cost-efficient for the production of electricity and water supply, and illustrates perfectly the rational use of water resources. In the past decades, hydroelectric power was one of the main sources of energy from water. Today, the water sector is considered a direct consumer of electricity, and this affects the distribution of water resources². Active energy consumption directly affects the state of water resources in the world, which in turn is the cause of climate change. Rational use of energy should contribute to sustainable development through the competent distribution of energy resources at all stages of transformation. For a comprehensive assessment of the problem, an example of the construction of the SHPP on the Arzakan —Yerevan water pipeline is given.

Materials and Methods. The Arzakan-Yerevan water pipeline supplies water to the administrative districts of Yerevan (Arabkir, Malatia-Sebastia, Achapnyak, Erebuni) and adjacent settlements (Zovuni, Kanakeravan, Nor-Achin, Nor-Gekhi, Yeghvard, etc.). The water pipeline originates from the Arzakan spring, and on the territory of the Getamej administrative community, it is divided into two water pipelines with different pressure conditions. The construction of the SHPP on one of the branches of this water pipeline feeding the north-western and western districts of Yerevan is being considered. The company operating on the dividing branch of the water pipeline installed a semi-open valve to adjust the water flow in the system. This provides the pressure in the water supply for settlements located above the main highway (adjacent to the city of Nor-Achin). When the valve is semi-open, a local hydraulic resistance of about 13 m of water column (w.c.) is created (before that, the pressure in the water pipe was about 33 m w.c.).

We propose to install a SHPP instead of a semi-open valve, without disturbing the normal hydraulic mode of operation of the water supply system. Figure 1 shows a plan of the area in the Google Earth program, and Table 1 summarizes the actual values recorded by the continuous pressure recording device (logger) for 12 hours.

The expediency of the practical use of the so-called “hidden” energy generated in the sections of main water pipelines in the Republic of Armenia should be noted. The hydropower potential of water supply systems has long been known, but not properly explored. The literature describes in detail the possibilities of using microturbines to generate electricity in water supply systems [3−5]. Gravity systems in areas with high slopes form significant pressures in water supply and distribution networks, creating the prerequisites for generating hydroelectric power. Turbines installed in water distribution networks can also be used as pressure control systems. To do this, pressure relief valves (PRV), which control water losses and leaks, are used [6]. They dissipate energy, thus reducing the pressure in the system. As for water turbines, they can convert excess pressure into electricity³. Major advantages of hydraulic energy recovery in water pipes, according to F. Vieira and J. S. Ramos [3][7], are improving the energy efficiency of the system through the use of local sources, and reducing dependence on external (grid) energy. In addition, hydraulic energy recovery contributes to an overall reduction in operating costs. F. Vieira and J. S. Ramos [3][7] emphasize in their research that the use of small hydroelectric power plants in water supply systems reduces significantly the cost of their operation. This is due to the fact that the proposed solution enables to replace the control valve with a hydraulic turbine equal in hydraulic resistance. Table 2 shows the water consumption in the system (monthly averages are indicated).

Note that the maximum head is Hmax = 21.4 m, the minimum — Hmin = 20.3 m.

With a design head equal to H = 20 m, the flow power at the turbine inlet [12]:

Principles for selecting a hydro turbine. With a low head and a higher flow rate (for SHPP), a turbine with a high specific speed should be selected. Of the reaction turbines, an axial jet turbine will do. Of the low-power active turbines, Banki turbine should be chosen to pass a relatively large flow rate. However, in this case, it is unacceptable to use an active turbine, since when it is filled, excess pressure is formed at the outlet, and Banki turbine operates as a very poor jet turbine with very low energy performance.

It is recommended to install the hydro turbine unit at the valve on the bypass line. Inlet and outlet valves will provide uninterrupted operation of the line even in the event of a station shutdown. The operation of the SHPP will be monitored by pressure sensors installed at the inlet and outlet of the turbine. This maintains the required pressure on the line⁴. When installing equipment on drinking water mains, special requirements should be taken into account [8-10]. Specifically, contamination of the water with lubricating oils and other materials^{5, 6} must be avoided. In general, the distributed water should meet the quantitative and qualitative standards of drinking, domestic, and industrial consumption [11].

Energy indicators of SHPP. The following average efficiency values are applied for different elements of the hydro turbine installation:

Turbine shaft power:

and at the generator output:

Table 3 presents the averaged parameters of the power of the SHPP and the corresponding indicators of the average monthly electricity generation.

It should be noted that the use of a SHPP jet turbine at a large negative suction height (h_S = –13 m), in our opinion, is proposed for the first time in world practice. In this case, on the one hand, the occurrence of cavitation is excluded [9][12], which, undoubtedly, is regarded positively. On the other hand, it is not known how the high pressure at the outlet of the turbine affects the efficiency, therefore, in our calculations, a slightly underestimated indicator is used.

Research Results. The electricity produced by the SHPP (Table 3) is planned to supply to the general energy system of the Republic of Armenia on the terms of guaranteed purchase at prices approved by the Commission on Regulation of Public Services of Armenia (currently, the tariff is 10.579 drams per kWh).

According to Table 3, during the year, the SHPP will produce at least 1.5 million kWh. According to preliminary calculations, the construction period of the SHPP is 24 months, and the service life is 30 years. The cost of construction and design of the SHPP will amount to 60.4 million drams (Table 4).

Anticipated revenues, operating costs, and profit are shown in Table 5.

The data on annual expenses presented in Table 5 are based on the current regulatory legal acts of the Republic of Armenia. According to these documents, the value added tax and income tax is 20 %⁷, and the property tax is 0.6 % (of the residual value). Based on the data on the construction and current operating expenses of the SHPP, Table 6 shows the key indicators of financial productivity.

Discussion and Conclusions. Due to the mountainous terrain in the greater territory of the Republic of Armenia, local resistance is often created to adjust the pressure in the water supply network. As a rule, we are talking about semi-open valves and the installation of pressure control equipment. Equipping the network nodes with specially designed turbines will enable converting artificially created local resistance and extinguished mechanical energy into electrical energy. It should be noted that more than 300 semi-open valves and pressure control equipment were installed in the Yerevan water supply system alone. Many of them require replacement. The solutions proposed in this article can be applied to the areas under repair.